1. Field of the Invention
This invention relates to the field of printed circuit board manufacture.
2. Prior Art
There are many methods of manufacturing printed circuit boards used extensively throughout the electronics industry. The advent of large scale integrated circuits ("LSI") and very large scale integrated circuits ("VLSI") has created an ever increasing demand for a fast, economical way of mass producing printed circuit boards which are ready for component connections.
One method of mass producing printed circuit boards entails forming preselected paths on prefabricated circuit boards with conductor pathways affixed thereon such as is disclosed in Chiaretta, U.S. Pat. No. 3,584,183. In Chiaretta, a first set of parallel conductors is disposed on a circuit board by standard fabrication techniques so that it is electrically connected to the diode elements fabricated within the board. A second set of parallel conductors is disposed so as to cross the first set of conductors. The connections between the conductors and the diodes are selectively removed to provide the desired circuit configuration. Such selected removal of connections is achieved by selectively bombarding these connections with a pulsed laser beam to burn them away. To accomplish the removal, the patent discloses an automatically controlled movable table with high precision indexing, a small fixed laser, and a transport logic circuit which receives input information from a standard input system (such as a tape transport, card reader, etc.)
Another method of mass producing printed circuit board is disclosed in Winter et al, U.S. Pat. No. 3,148,438. In Winter, a regular pattern of strips of electrically conductive material is bonded on one side of a insulative material board. The board is provided with a multiplicity of regularly distributed apertures which extend at spaced intervals in a grid pattern through the conducting strips. The circuit is constructed by inserting the terminals of circuit components through the appropriate apertures on the opposite side of the board and soldering the terminals to the conductor strips. The conducting strips are interrupted where necessary to establish the conductive pathways by utilizing a manual cutting tool such as a spotface cutter. As an alternative, the conductive strips may be interrupted prior to affixing the circuit components.
A further method of mass producing printed circuit boards is disclosed in Goodwin Jr. et al, U.S. Pat. No. 3,226,802. In Goodwin, premade copper conductor pathways are fabricated on both sides of a board, apertures are made in the board and are filled with conductive material to electrically interconnect the conductors on each side of the board. The apertures are selectively broken according to a preselected circuit pattern by a manually operated machine.
In all of the prior art patents, the connections between the conductors are selectively interrupted either by mechanically breaking a conductor path or by utilizing a laser to cut or burn the conductor connections away. Each interruption occurs one at a time. Thus, for applications where there may be over 24,000 interruptions on one board, removal of the conductor connections may take up to several months to complete. Further, in LSI and VLSI circuit applications, the conductor pathways are extremely thin and close together. Therefore, it would be quite difficult to burn interruptions in the conductors patterns and maintain a high degree of reliability even with the aid of a microscope and a very skilled technician. Again, this process would take extremely long periods of time to complete.
In applications where the printed circuit boards are attached to each other to form a multilayer printed circuit board, provisions must be made to connect the pathways of the various boards which are stacked together. In the prior art, apertures are formed in the printed circuit board which connect to each of the conductors. The apertures are plated through to connect conductors on one surface with conductors on the surface of an adjacent board. However, in some applications, it may be desirable to "jump around", i.e., connect alternative layers and/or alternative surfaces anywhere in the stack. In the prior art when the apertures in each board are plated through there will always be a connection with the conductors bordering the hole. Thus, it would be impossible to alternate the connections between different boards.
In the fabrication of multilayer circuit boards, the reliability of each layer is extremely crucial. Therefore, each layer must be inspected for accuracy. Typically, inspection in this field is rather slow and labor intensive since it is done visually by a skilled technician. It is quite common to spend up to four to eight hours inspecting one layer of 18 that are needed to make an LSI or VLSI multilayer circuit board.